The proposed experiments are designed to investigate the role of GTP and GTP-binding proteins in the process of visual adaptation in vertebrate rods. Intracellular recordings will be made in the isolated retina preparation of the toad, Bufo marinus. Responses to light of dark- and light-adapted rods will be recorded both before and after injections of compouonds known to affect GTP-binding regulatory proteins in other systems. The primary goal of these experiments is to determine the possibility of affecting responsiveness of light-adapted rods by intracellular injection of hydrolysis-resistant analogs of GTP. Experiments will be performed to see if rod responsiveness is governed (in part) by a GTP-binding regulatory protein that has properties similar to the guanyl nucleotide binding component of hormone-stimulated adenylate cyclase. The results of this preliminary study will determine the potential for the use of these techniques in future studies of molecular mechanisms which act at the cellular level during light- and dark-adaptation.